Taming mavericks: Researchers use synthetic magnetism to control light

October 31, 2012
by Andrew Myers

(Phys.org)—Stanford researchers in physics and engineering have demonstrated a device that produces a synthetic magnetism to exert virtual force on photons similar to the effect of magnets on electrons. The advance could yield a new class of nanoscale applications that use light instead of electricity.

Magnetically speaking, photons are the mavericks of the engineering world. Lacking electrical charge, they are free to run even in the most intense magnetic fields. But all that may soon change. In a paper published in Nature Photonics, an interdisciplinary team from Stanford University reports that it has created a device that tames the flow of photons with synthetic magnetism.

The process breaks a key law of physics known as the time-reversal symmetry of light and could yield an entirely new class of devices that use light instead of electricity for applications ranging from accelerators and microscopes to speedier on-chip communications.

"This is a fundamentally new way to manipulate light flow. It presents a richness of photon control not seen before," said Shanhui Fan, a professor of electrical engineering at Stanford and senior author of the study.

A DEPARTURE

The ability to use magnetic fields to redirect electrons is a founding principle of electronics, but a corollary for photons had not previously existed. When an electron approaches a magnetic field, it meets resistance and opts to follow the path of least effort, travelling in circular motion around the field. Similarly, this new device sends photons in a circular motion around the synthetic magnetic field.

The Stanford solution capitalizes on recent research into photonic crystals – materials that can confine and release photons. To fashion their device, the team members created a grid of tiny cavities etched in silicon, forming the photonic crystal. By precisely applying electric current to the grid they can control – or "harmonically tune," as the researchers say – the photonic crystal to synthesize magnetism and exert virtual force upon photons. The researchers refer to the synthetic magnetism as an effective magnetic field.

The researchers reported that they were able to alter the radius of a photon's trajectory by varying the electrical current applied to the photonic crystal and by manipulating the speed of the photons as they enter the system. This dual mechanism provides a great degree of precision control over the photons' path, allowing the researchers to steer the light wherever they like.

BROKEN LAWS

In fashioning their device, the team has broken what is known in physics as the time-reversal symmetry of light. Breaking time-reversal symmetry in essence introduces a charge on the photons that reacts to the effective magnetic field the way an electron would to a real magnetic field.

For engineers, it also means that a photon travelling forward will have different properties than when it is traveling backward, the researchers said, and this yields promising technical possibilities. "The breaking of time-reversal symmetry is crucial as it opens up novel ways to control light. We can, for instance, completely prevent light from traveling backward to eliminate reflection," said Fan.

The new device, therefore, solves at least one major drawback of current photonic systems that use fiber optic cables. Photons tend to reverse course in such systems, causing a form of reflective noise known as backscatter.

"Despite their smooth appearance, glass fibers are, photonically speaking, quite rough. This causes a certain amount of backscatter, which degrades performance," said Kejie Fang, a doctoral candidate in the Department of Physics at Stanford and the first author of the study.

In essence, once a photon enters the new device it cannot go back. This quality, the researchers believe, will be key to future applications of the technology as it eliminates disorders such as signal loss common to fiber optics and other light-control mechanisms.

"Our system is a clear direction toward demonstrating on-chip applications of a new type of light-based communication device that solves a number of existing challenges," said Zongfu Yu, a post-doctoral researcher in Shanhui Fan's lab and co-author of the paper. "We're excited to see where it leads."

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A mirror would be more akin to an electric field, which deflects electrons.

Using this new technology to 'corral' light into solar cells would add another layer of expense and would not fly economically.

It might be used for volumetric (3D) displays but would add an internal layer and would have to be a line display, one such device per line of video. It is easy to imagine but the engineering details could kill you. If I get it right, the light is deflected to any path in the same plane as the original light, so it basically would turn left or turn right at some given angle.

So the chip would need to be sticking out perpendicular to the plane of the screen. It would depend on how small such a chip could be made and if it could be integrated into the basic design of the overall screen.

Using multiple devices in various arrangements or other redesigns looks quite promising. Other designs just might work in solar. With this technology is seems information could be stored in a vortex of light, as opposed to driving a motor with electricity. It further seems that the lack of generating force would allow for substantial amounts of information to be stored and retrieved at light speed.

"It cannot go back" Would this also be usable as a photonic diode? Two diodes back to back make a transistor, so if it is, they could be very close to a "simple" photonic transistor, and with that, a practical photonic computer.

Shame on Stanford researchers.They don't know what a refraction is. Photonic crystal is a crystal with alternating index of refraction. The light propagates the way it has extreme optic length. So it will bend in different media or in a media with alternating index of refraction. And it is NOT "effective magnetic field"!What did they do? They took a piezoelectric photonic crystal, and by applying electric current they managed to change refraction index, so to say bend the light.What is wrong with a "broken" law? We have maxwell's laws which are pretty invariant to time-reversal symmetry.

The crystal device allows "harmonic tuning." In other words,it creates synchronized oscillations that can be tuned at different frequencies. Call this the "A" stream of oscillations. Synthetic magnetism--not even organic to Mother Nature. The other control knob "manipulat[es] the speed of the photons as they enter the system." Again, synchronized oscillations, at varying frequencies. The "B" stream of oscillations. The A units are identical and the B units are identical. The A and B streams can be tuned at different oscillation levels by their control knobs. Art Winfree's law of coupled oscillators, circa 1967, strongly implies that the B stream (the minor "force") will behave differently due to interactive effects as A-B varies. See also Kuramoto, Strogatz, Mirollo. All four ply their trade in different fields, but their ideas may be adaptable to put this result in a new "light."